1. Field of the Invention
The present invention relates to digital electronic systems, and, in particular, to circuitry used in digital computers and the like. More particularly, the invention relates to the suppression of radiated electrical noise generated by high frequency signals of such systems.
2. Description of the Related Art
The circuitry of digital electronic systems is typically mounted on printed circuit boards. For example, in small business or personal computers, such circuit boards include a main system board (often referred to as a mother board) and a number of smaller circuit boards that are inserted into connectors on the mother board. Typically, such circuit boards are multi-layer circuit boards. That is, the connections to and the interconnections between integrated circuits and other components on the surface of a circuit board are provided by metallic traces on the surface layers of the circuit board as well as on additional traces on inner layers of the circuit board. In addition, exemplary multi-layer printed circuit boards include layers that are dedicated to a particular voltage reference. For example, the ground reference (i.e., zero volts) may be distributed on an inner layer of the circuit board. In addition, the primary supply voltage (e.g., +5 volts) may also be distributed on a separate dedicated layer of the circuit board. Interconnections between the layers of the circuit board are provided by feedthrough connections (often referred to as vias). In particular, connections between integrated circuits on one surface layer and circuit paths on the opposite surface layer are provided by vias. Similarly, the power and ground connections to the integrated circuits are provided by vias that are electrically connected to the respective voltage reference layers.
Digital electronic circuits generally operate in synchronism with one or more clock signals that are distributed to the integrated circuits on the printed circuit boards. For example, small business/personal computers, such as computers that utilize the Intel.RTM. 80.times.86 family of microprocessors utilize a clock signal that is a multiple of the rated operating frequency of the microprocessor. As a specific example, a computer based upon an Intel.RTM. 80386 microprocessor operating at 25 MHz utilizes a system clock signal having a frequency of 50 MHz. Such an operating frequency is close to the lower end of the television broadcast band which begins at 54 MHz. If the clock signal is permitted to radiate significant energy, it may interfere with the operation of television receivers and other radio frequency communications equipment. Such radio frequency interference (RFI) is not permitted by regulations promulgated by the Federal Communications Commission (FCC). Thus, it is necessary to suppress the radiation of electromagnetic noise from the clock signal and other signals operating at high frequencies in order to comply with FCC regulations.
One method of suppressing the radiation of electromagnetic energy from a digital computer is to shield the entire computer within a metallic enclosure. Such metallic enclosures serve the dual purpose of providing electromagnetic shielding as well as providing structural support for circuit boards, disk drives and other components within the computer. However, this method only suppresses direct radiation to the external environment of the computer and does not prevent radiation of such noise to other signal lines within the computer. Since it is generally necessary to provide signal lines to interconnect a computer to an external device, such as a printer or the like, the noise generated by a high frequency signal may be picked up on a signal line and thus radiated externally to the computer.
In systems where the high frequency clock signals are localized on particular cards or on particular portions of a single card, the high frequency clock circuitry on the cards or card portions can be shielded from other circuits using conventional shielding techniques such as surrounding the cards or the card portions with a metallic enclosure. Again, this is effective only if signal lines carrying the high frequency clock signals can be isolated from other signals that pass beyond the boundaries of the shielding enclosure. Furthermore, the mechanical shielding structures are generally bulky and add to the overall size of the computer system in which they are used.
Another method that has been utilized is to provide the shielding as part of the printed circuit board. For example, the high frequency clock lines can be positioned on an inner layer of a circuit board proximate to one or two voltage reference layers. The voltage reference layer or layers operate to shield the clock lines to suppress the radiation of electromagnetic energy. This technique provides significant improvement over the previously described techniques since the shielding is applied directly to the printed circuit boards having the high frequency clock lines. Although this technique provides substantial improvement in the suppression of emitted RFI from the clock lines, one skilled in the art will appreciate that an additional layer of the printed circuit board increases the cost of the circuit both in terms of material as well as engineering and manufacturing costs. In addition, the additional layer increases the thickness of the circuit board. The additional thickness may not be acceptable, particularly when the printed circuit board has an edge connector formed on it. Such an edge connector may be too large to insert into a conventional card edge connector used in typical computers. Thus, a need exists for a RFI suppression technique compatible with the existing standards for printed circuit boards.